A variety of optical structures have been proposed in order to imitate iridescent colors occurring in nature, for example, in wings of the Morpho butterflies and feathers of some birds such as the hummingbirds.
A three-dimensional nanostructure shown in FIG. 1 simulates wing scales of the Morpho butterfly; the nanostructure has been disclosed by Watanabe et al. in Brilliant Blue Observation from a Morpho-Butterfly-Scale Quasi-Structure, Japanese Journal of Applied Physics, Vol. 44, pp. L48-L50, 2005. The structure has been fabricated using focused-beam chemical vapor deposition (FIB-CVD) technique, and the desired color effect has been observed under a microscope. However, the Morpho butterfly wings contain multiple structures in close proximity to each other, as shown in FIG. 2 from Photophysics of Structural Color in the Morpho Butterflies by Kinoshita et al., FORMA, 17, pp. 103-121, 2002, and the Watanabe's structure resembles only one of them. This technique is complicated and it would be extremely difficult, if not impossible, to produce multiple side-by-side structures using the method used by Watanabe et al. in manufacturing the isolated three-dimensional structure.
U.S. Patent Application No. 20060002656 published Jan. 5, 2006, in the name of Cowan et al., teaches stepped surface relief optical structures shown in FIG. 3, which produce a narrow band of color by resonance or constructive interference of reflected light. The stepped structures 94 are known as Aztec structures and can be formed by holographic techniques, and then replicated onto a continuous film 92. However, use of the film with stepped structures for security/authentication purposes is limited by the fact that the film is manufactured separately and then bonded to a substrate, such as a secure document, therefore counterfeiters can obtain the film directly from its manufacturer or detach it from a valid document.
R. Coath in Investigating the Use of Replica Morpho Butterfly Scales for Colour Displays, Univ. of Southampton, IRP report, pp. 1-9, 2007, teaches depositing multiple layers of materials having different indices of refraction on a silicon wafer, followed by electron beam lithography and selective etching of the multilayer design to create a tree-like structure similar to the wings of the Morpho butterfly. This technique is also complicated and costly.
An object of the present invention is to provide an iridescent coating overcoming the shortcomings of the prior art and a method of manufacturing thereof.